Lazy Tach with 280ZX Distributor

[cgsheen1]

Pretty much everyone I know here in Phoenix with a carbed 240Z or L-series is now using the 280ZX "matchbox" distributor with their stock tach and no one I know complains about it. There was an individual here famous for his recommendation of that particular swap. I'm NOT a carb guy - gave that up in the '80's - but I been running with these guys for more than 12 years all over the Arizona countryside. I believe I can attest to the fact that the stock 240Z tach will work with a non-point distributor. It'll be interesting to learn why yours will not...

Edited September 11, 2024 by cgsheen1

[kcpope]

Thanks Steve. If I do the swap, I’ll adapt the 260z tach guts into the 240z housing to maintain mounting points and the correct font as zed mentioned.

But before I do any of that, I’ll try pulling the existing tach out and see if I can see any obvious problems.

As I’m thinking about it more, I’d be suprised if the e12-80 is failing. The engine seems to run too good for that to be the case.

PS: the whole reason I needed the tach was to sort out my vacuum advance timing as I mentioned in my other thread. I went ahead and bought a cheap optical rpm gauge on Amazon that I’ll use in the meantime. At least I can get my timing properly setup, then move on to fixing the tach.

[kcpope]

6 minutes ago, cgsheen1 said:

Pretty much everyone I know here in Phoenix with a carbed 240Z or L-series is now using the 280ZX "matchbox" distributor with their stock tach and no one I know complains about it. There was an individual here famous for his recommendation of that particular swap. I'm NOT a carb guy - gave that up in the '80's - but I been running with these guys for more than 12 years all over the Arizona countryside. I believe I can attest to the fact that the stock 240Z tach will work with a non-point distributor. It'll be interesting to learn why yours will not...

Agreed, I’ll do my best to get to the bottom of it. I’d love to not have to swap tachs. We’ll see.

[cgsheen1]

I also had it in the back of my mind that the stock coil was 1.5 Ohm. I just looked at the FSM: Primary - 1.5 - 1.7 Ohm at 20 degrees C...

[kcpope]

9 minutes ago, cgsheen1 said:

I also had it in the back of my mind that the stock coil was 1.5 Ohm. I just looked at the FSM: Primary - 1.5 - 1.7 Ohm at 20 degrees C...

Yeah, I’ve read mixed things regarding the coils. As Zed mentioned earlier, the 280zx dizzy wants to see around 1 ohm resistance, which is exactly what I have. Did the ZX also have a ballast resistor?

In general when people convert a 240z from points to electronic ignition, here’s what I’ve read:

if you’re using a 1.5 ohm coil, add a 1.5 ohm resistor as well.

if you’re using a 3.0 ohm coil, no resistor needed.

This seems to be the case if you’re doing a pertronics ignition, but I’m not sure if the same applies to a 280zx dizzy…

[ollie]

I don't believe the 280zx had a ballast resistor.  The coil I'm using is the ngk 48776 which supposedly measures out at .93 ohms.  I should measure mine...

[kcpope]

4 minutes ago, ollie said:

I don't believe the 280zx had a ballast resistor.  The coil I'm using is the ngk 48776 which supposedly measures out at .93 ohms.  I should measure mine...

That’s within spec for your primary. What’s your secondary ohm reading?

also, are you running the little stock condensers? One at the coil. One near the alternator. I’m going to try adding mine back in and see if it makes a difference.

 

[Zed Head]

Here's an experiment you could try if you just want to mess around and try things.  It fits my earlier comment about too much current maybe screwing up the tach function (even though many people have done the conversion with no problems).

Run a parallel power wire to the coil.  This will reduce current through the tach sensor loop.  You can run a jumper, then if it works, just run a permanent bypass wire.

Overall though, considering cgsheen's comment about all of the people he knows with no problems, it might just be that the tach components are wearing out.  There are many posts about flakey (flaky?) tachs.  My 76 tach flaked out in the first few years after I brought the car back to life.  It also had problems after the GM HEI module conversion even though it was already designed for electronic ignition.  Electronics components weren't as high volume and consistent in quality in the 70's as they are today.  Today's light bulbs have printed circuit boards inside.  It's crazy.

Anywyay, this shouldn't hurt anything.  Not sure why that red line came out pink.

p.s. you can just run a wire directly from the battery terminal for the test.  The engine will keep running though until you remove it.  Hot wire.

 

Edited September 11, 2024 by Zed Head

[ollie]

1 hour ago, kcpope said:

That’s within spec for your primary. What’s your secondary ohm reading?

also, are you running the little stock condensers? One at the coil. One near the alternator. I’m going to try adding mine back in and see if it makes a difference.

 

I have the one at the coil but not the alternator.  The pin snapped off last time I was tinkering and I haven't added it back.  I thought it was only for radio noise suppression?  I may have invented that point however. 

Edited September 11, 2024 by ollie

[kcpope]

Yeah, radio interference, I’ve heard that too. I figured that same interference might be messing with the tach. It’s a long shot, but at this point I figure, why not 😅

[ollie]

6 minutes ago, kcpope said:

Yeah, radio interference, I’ve heard that too. I figured that same interference might be messing with the tach. It’s a long shot, but at this point I figure, why not 😅

Exactly, fingers crossed!   That'd be amazing, and could account for when my tach works perfectly to 4,000 rpms then basically hangs there irrespective of engine speed.  Worked great before and I have to say this distributor is a fantastic upgrade.  Even with carbs being hot or cold, it bang starts all the time.  

[kcpope]

Still fiddling around with it…

Does anyone know what this solid black wire is for? Comes out of the factory wire harness at the coil. It’s the only wire  not connected to anything for me.

Maybe the ground wire for the old points distributor?